"Permanent TSB, which is owned by Irish Life & Permanent, has been under pressure due to the credit crunch.
It is losing money in its tracker loans and trying to make up the shortfall by increasing rates elsewhere.
People on standard variable rates will see rates increase from 2.69% to 3.19% from Monday.
More than 50,000 borrowers will be affected.
The move will mean monthly repayments on a €300,000 mortgage will increase by €70 a month.
Most of its customers who took out mortgages over the past five years are on tracker loans and will not be affected by the move.
Bank of Ireland, Ulster Bank, Halifax Bank of Scotland and KBC Homeloans say they have no immediate plans to follow suit but will be watching the move closely."